Heat Exchanger Element and a Water Heater and Heat Pump Utilizing Same

ABSTRACT

A heat exchanger element ( 30 ) formed from an inner copper tube ( 32 ) and an outer copper tube ( 34 ). The entire outer surface of the inner tube ( 32 ) is substantially in contact with the entire inner surface of the outer tube ( 34 ).

FIELD OF THE INVENTION

The present invention relates to a heat exchanger element and a water heater and a heat pump utilising same.

The invention has been primarily developed for use in domestic hot water heaters and will be described hereinafter with reference to this application. However, the invention is not limited to this particular use and is also suitable for heating fluid other than water in numerous heat exchanger applications.

BACKGROUND OF THE INVENTION

Regulatory authorities in many jurisdictions, including Australia, Europe and the United States of America, require heat exchanger elements used in domestic hot water heaters to have two layers of metal separating the heated fluids circulating through the heat exchanger and the water being heated in the hot water tank. This is to ensure that even if one of the layers is compromised, there will be no mixing between the heat exchanger fluid and the water supplied to user.

One known ‘double walled’ hot water heater utilises a copper tube wrapped around the outside of a cylindrical steel water tank. Heat exchanger fluid is pumped through the copper tube. However, this arrangement is difficult and thus relatively expensive to produce, particularly the step of rolling the tube around the approximately 600 mm diameter tank. This arrangement also has relatively poor heat exchanging efficiency, as steel is almost 30 times less effective as a conductor than copper.

Another known ‘double walled’ hot water heater utilises a coil of copper tube positioned inside a tube within a water tank. This arrangement is also difficult and thus relatively expensive to produce. It is difficult to ensure the copper tube makes good surface to surface contact with the tube interior, which is required for good heat exchanging efficiency.

OBJECT OF THE INVENTION

It is the object of the present invention to overcome or substantially ameliorate at least one or more of the above prior art deficiencies.

SUMMARY OF THE INVENTION

Accordingly, in a first aspect, the present invention provides a heat exchanger element formed from an inner copper tube and an outer copper tube, wherein the entire outer surface of the inner tube is substantially in contact with the entire inner surface of the outer tube.

In one form, the inner diameter of the outer tube is substantially equal to the outer diameter of the inner tube, such that approximately 100% of the inner surface of the outer tube is in contact with the outer surface of the inner tube.

In another form, the inner surface of the outer tube includes rifling or grooves having a minimum diameter substantially equal to the outer diameter of the inner tube, such that approximately 80% of the inner surface of the outer tube is in contact with the outer surface of the inner tube.

In an embodiment, the element is preferably substantially cylindrical in shape, and most preferably with cylindrical walls formed from spiralled inner and outer tubes.

In another embodiment, the element is preferably substantially cylindrical in shape, with cylindrical walls formed from spiralled inner and outer tubes, and includes an outer cylindrical wall attached to the exterior of the spiralled outer tubes.

In yet another embodiment, the element is preferably substantially cylindrical in shape, with cylindrical walls formed from spiralled inner and outer tubes, and includes an inner cylindrical wall attached to the interior of the spiralled outer tubes.

In yet another embodiment, the element is preferably substantially cylindrical in shape, with cylindrical walls formed from spiralled inner and outer tubes, and includes inner and outer cylindrical walls respectively attached to the interior and exterior of the spiralled outer tubes.

The element preferably has two substantially longitudinal inlet and outlet portions, one extending from a first end of the cylinder and the other extending from the second end of the cylinder through the cylinder interior to the first end of the cylinder.

In a second aspect, the present invention provides a water heater including a tank and a heat exchanger element according to the first aspect.

In a third aspect, the present invention provides a method of forming a heat exchanger element, the method comprising the following steps:

-   a. drawing a first and a second copper tube, the first tube having     an outer diameter less than the internal diameter of the second     tube; -   b. inserting the first tube into the second tube; -   c. passing the tubes through a roll former adapted to crush the     second tube onto the first tube, such that the entire outer surface     of the first tube is substantially in contact with the entire inner     surface of the second tube. -   d. annealing the crushed tubes; -   e. bending the tubes into a desired configuration; and -   f. annealing the bent tubes.

Step e. preferably includes bending the tubes into a generally cylindrical shape, most preferably with the cylindrical walls being formed from spiralled tubes.

Step a. preferably includes drawing the second tube with internal rifling or grooves.

In a fourth aspect, the present invention provides a hot water heat pump comprising:

-   a heat pump circuit comprising an evaporator coil; a compressor, a     condenser coil in a heat exchanging relationship with a body of     water, and a thermostatic expansion valve (TX valve), the circuit     adapted to hold a charge of refrigerant, -   wherein the condenser coil is in the form of a heat exchanger     element according to the first aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described, by way of examples only, with reference to the accompanying drawings, wherein:

FIG. 1A is a schematic cross-sectional side view of a first embodiment of a water heater according to the invention, the water heater having a first embodiment of a heat exchanger element according to the invention;

FIG. 1B is a cross-sectional end view of the heat exchanger element shown in FIG. 1A;

FIG. 2 is a schematic cross-sectional side view of a second embodiment of a water heater according to the invention

FIG. 3 is a schematic cross-sectional side view of a third embodiment of a water heater according to the invention;

FIG. 4 is a schematic cross-sectional side view, of a fourth embodiment of a water heater according to the invention;

FIG. 5 is a cross-sectional end view of a second embodiment of a heat exchanger element according to the invention; and

FIG. 6 is a schematic view of an embodiment of a hot water heat pump according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1A shows a first embodiment of a domestic hot water heater 10 according to the present invention. The water heater 10 includes a 250 litre volume, insulated steel cylindrical tank 12 with upper and lower domed ends 14 and 16 respectively. The tank 12 is mounted with its longitudinal axis substantially vertical. The upper domed end 14 has a central necked opening 18 with an external flange 20. In use, the tank 12 is almost completely filled with water.

The tank 12 also has a first hot water outlet 22 within the upper domed end 14 and a first cold water inlet 24 in the lower domed end 16. The outlet 22 is, in operation, connected to household plumbing. The inlet 24 is, in operation, connected to mains water supply. The tank 12 also has a second hot water outlet 26 and a second cold water inlet 20 which, for example, can be used to connect to the tank 12 in parallel to a solar hot water heater.

A first embodiment of a heat exchanger element 30 is inserted into the tank interior through the opening 18. As best shown FIG. 1B, the element 30 is formed from inner and outer copper tubes 32 and 34 respectively. The outer diameter of the inner tube 32 is substantially equal to the inner diameter of the outer tube 34, such that the entire outer surface of the inner tube 32 is substantially in contact with the entire surface of the outer tube 34. The inner and outer surfaces being in substantial contact maximises the transfer of heat from hot refrigerant fluid (e.g. water or glycol or a mixture thereof) 36 pumped through the interior of the inner tube 32 to the water in the tank 12. The heat exchanger element 30 also has an inlet portion 38 and an outlet portion 40. The circulation of the water in the tank 12 caused by the energised heating element 30 is indicated by the arrows. One advantage of the water heater 10 is it relatively inexpensive and simple to manufacture due to the introduction of the element 30 into the tank 12 via the opening 18. Another advantage is relatively high efficiency due to the direct refrigerant fluid to water heat transfer via the copper tubes 32 and 34, which have relatively high conductivity.

The construction of the heating element 30 involves the following steps. Firstly, two copper tubes are drawn to approximately 19 metres in length. The smaller diameter tube 32 has an internal diameter of 5.0 mm and an external diameter of 5.5 mm. The large diameter outer tube 34 has an internal diameter of 5.7 mm and an external diameter of 6.3 mm. The difference between the outer diameter of the inner tube 32 and the inner diameter of the outer tube 34 allows the inner tube 32 to be inserted within the outer tube 34. At this stage, there is a small gap between the outer surface of the inner tube 32 and the inner surface of the outer tube 34. The two tubes 42 and 34 are then fed through a roll former which crushes the outer tube 34 into contact with the inner tube 32 to eliminate the gap. The roll forming also slightly extends the length of the outer tube 34 and, if desired, it can initially be made slightly shorter than the inner tube 32 so that it expands to a common length. Alternatively, the excess of the outer tube 34 can be trimmed. The roll forming also hardens the tubes 32 and 34. The (roll formed/crushed) tubes 32 and 34 are then annealed for softening, so they can then be bent and spiral wound into the shape shown in FIG. 1. The (bent/spiralled) tubes 32 and 34 are then annealed again. Finally, appropriate connection fittings (e.g. brass) and then attached to the inlet portion 38 and outlet portion 40.

FIG. 2 shows a second embodiment of a water heater 210. Like features to those used to describe the first embodiment are indicated using like reference numerals. The water heater 210 is very similar in construction and operation to the water heater 10 shown in FIG. 1, except the heating element 30 is enclosed by an external cylinder or tube 42 which serves to increase the conduction area of the heat exchanger element 30 and thus increase the heat transfer from heated fluid 36 to the water in the tank 12.

FIG. 3 shows a third embodiment of a water heater 310. Like features to those used to describe earlier embodiments are indicated using like reference numerals. The water heater 310 is also very similar in construction and operation to the water heater 10 shown in FIG. 1, except the heat exchanger element 30 has an internal cylinder or tube 44 to increase its conduction area.

FIG. 4 shows a fourth embodiment of a water heater 410. Like features to those used to describe earlier embodiments are indicated using like reference numerals. The water heater 410 is also very similar in construction and operation to the water heater 10 shown in FIG. 1, except the heater exchanger element 30 has external and internal tubes 42 and 44 respectively, to increase its conduction area.

FIG. 5 shows a second embodiment of a heat exchanger element 130 formed from inner and outer copper tubes 132 and 134 respectively. The element 130 is constructed in a similar manner as that described previously in relation to the element 30, except the inner surface of the outer tube 134 is initially provided with internal rifling or grooves 136. After roll forming, only approximately 80% of the inner surface of the outer tube 134 is in contact with the outer surface of the inner tube 132 due to the gaps left by the rifling/grooves 136. This slightly reduces the conductivity/heat exchanging efficiency of the element 130. However, the air gaps are required to meet safety regulations in some jurisdictions.

FIG. 6 shows an embodiment of a hot water heat pump 140 according to the invention. The heat pump 140 has a heat pump circuit comprising an evaporator coil 142, a compressor 144, a condenser coil 146, and a thermostatic expansion valve (TX valve) 148. The circuit is configured with an outlet of the compressor 144 connected to an inlet of the condenser coil 146, an outlet of the condenser coil 146 connected to an inlet of the TX valve 148, an outlet of the TX valve 148 connected to an inlet of the evaporator coil 142 and an outlet of the evaporator coil 142 connected to an inlet of the compressor 144. The circuit is adapted to hold a charge of refrigerant. The components of the heat pump 140 operate to produce a refrigeration cycle, as is well known to persons skilled in the art. The condenser coil 146 is positioned in a heat exchanging relationship with a body of water contained by a tank 150. The condenser coil 146 is in the form of a double walled heat exchanger element as previously described with reference to FIGS. 1 to 5.

The advantage of the double walled heat exchanger element as the condenser coil 146 in the heat pump application is it provides the most direct, and thus efficient, transfer path of energy by conduction into the water. More particularly, as the condenser coil 146 is suspended in the water, there is no air gap around it. This results in a minimum coefficient of heat transfer from the refrigerant to the water.

This is in contrast to existing refrigerant systems which use copper piping systems to contain the refrigerant, such as existing heat pump configurations in which the copper pipe is wound on an external face of the water tank.

Further, the double walled construction of the condenser coil 146 provides a safety feature (being an internal refrigerant escape path between the inner and the outer tubes) that is only actuated when there is a failure in the inner tube. This safety feature is only possible due to the nature of the manufacturing process and the suspension of the condenser coil in water contained within a tank at a lower pressure than the refrigerant in the condenser coil.

Normal “tube in tube” designs have this safety feature permanently in place in the form of an air gap between the tubes, which results in poor heat transfer.

Although the invention has been described with reference to preferred embodiments, it would be appreciated by those persons skilled in the art that the invention may be embodied in many other forms. 

1. A method of forming a heat exchanger element, the method comprising the following steps: a. drawing a first and a second copper tube, the first tube having an outer diameter less than the internal diameter of the second tube; b. inserting the first tube into the second tube; c. passing the tubes through a roll former adapted to crush the second tube onto the first tube, such that the entire outer surface of the first tube is substantially in contact with the entire inner surface of the second tube. d. annealing the crushed tubes; e. bending the tubes into a desired configuration; and f. annealing the bent tubes.
 2. The method as claimed in claim 1, wherein step e. includes bending the tubes into a generally cylindrical shape.
 3. The method as claimed in claim 2, including the step of forming the cylindrical walls from spiraled tubes.
 4. The method as claimed in claim 1, wherein step a. includes drawing the second tube with internal rifling or grooves.
 5. A heat exchanger clement formed by the method as defined in claim
 1. 6. A water heater including a tank and a heat exchanger element according to claim
 5. 7. A heat pump circuit comprising an evaporator coil; a compressor, a condenser coil in a heat exchanging relationship with a body of water, and a thermostatic expansion valve (TX valve), the circuit adapted to hold a charge of refrigerant, wherein the condenser coil is in the form of a heat exchanger element according to claim
 5. 